Phase/matrix transformation weld process and apparatus

ABSTRACT

A welding method and apparatus are provided for forming a weld joint between first and second elements of a workpiece. The method includes heating the first and second elements to form an interface of material in a plasticized or melted state interface between the elements. The interface material is then allowed to cool to a plasticized state if previously in a melted state. The interface material, while in the plasticized state, is then mixed, for example, using a grinding/extruding process, to remove any dendritic-type weld microstructures introduced into the interface material during the heating process.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a division of application Ser. No. 09/994,506filed on Nov. 27, 2001.

ORIGIN OF THE INVENTION

[0002] This invention was made by employees of the United StatesGovernment and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to a welding method and apparatus,and in particular, to a welding method and apparatus which separatelyplasticizes or melts the surfaces to be joined followed by a subsequentweld matrix mixing process.

[0005] 2. Background of the Invention

[0006] Welding processes can be classified into one of two categories,fusion welding and solid state welding. Fusion welding involves meltingmaterial to be welded and includes such processes as MIG, TIG and VPPAwelding. Solid state welding joins materials without a melting step andinclude the processes of friction stir and inertia welding. Fusion weldprocesses typically result in a dendritic type weld microstructureexhibiting inferior mechanical and structural properties. Such inferiormaterial properties are generally seen in metals subsequent to melting.Conversely, solid state weld processes result in a non-dendritic grainstructure exhibiting properties superior to those produced with fusionwelding processes.

[0007] Both fusion welding and solid state welding have respectivelimitations. As indicted above, fusion welding compromises themicrostructure of the material and thus lessens the physical propertiesand characteristics of the material. Solid state welding such as inertiawelding is limited to rounded structures such as pipe or rod structures.

[0008] A recent advancement was made when friction stir welding becameavailable for the solid state welding of materials. Reference is made,for example, to U.S. Pat. Nos. 6,168,067 B1 to Waldron et al. and6,053,391 to Heideman et al. With the use of friction stir welding, asolid state weld could, for the first time, be provided in applicationsrequiring longitudinal welds, ranging from several inches to anunlimited length. As described in more detail in the aforementionedpatents, the friction stir weld process uses a rotating shoulder/pinconfiguration. The shoulder produces frictional heat to bring thematerial into a plasticized state and forges the welded material withextremely large forces. To accomplish the necessary large forces for theforging effect, a very robust backing anvil is required for support. Thewelding pin spins inside the workpiece at the same rate as the shoulder.The dependent motion of the welding pin and shoulder restricts the speedof the welding process.

BRIEF SUMMARY OF THE INVENTION

[0009] In accordance with the present invention, a welding method andapparatus are provided for forming a weld joint. The method separatesthe welding process into separate and discrete steps, one providingheating of first and second abutting elements to be welded to form aninterface therebetween of plasticized or melted material and a furthermatrix transformation step for processing the interface material aftercooling to a plasticized phase. The heating process may use anyconventional fusion welding process such as laser or plasma torch whichinitially melts the interface material. Subsequently, a separategrinding/extrusion element recrystallizes possible, undesirabledendritic matrix structures while the interface material is in theplasticized temperature state. Advantageously, separate heating sourcescan be used to plasticize or melt the respective first and secondelements forming the weld joint independently. The independent meltingfeature provides for the joining of dissimilar metals such as copper andaluminum or stainless steel and titanium.

[0010] According to one aspect of the present invention, a weldingmethod is provided for joining a workpiece comprising first and secondelements in abutting relation along facing surfaces. The method includesheating the first and second element to plasticize or melt the elementsat least at the facing surfaces so as to form an interface therebetweenof material in a melted state. If the interface material is heated to amelted state, then the interface material is allowed to cool from themelted state to a plasticized state. Next, the interface material ismixed while in the plasticized state.

[0011] According to another aspect of the present invention, anapparatus is provided for forming a weld joint in a workpiece betweenfirst and second elements in abutting relation along facing surfaces.The apparatus includes a heating device for plasticizing or melting thefirst and second elements at least at the facing surfaces so as to forminterface therebetween of material in a plasticized or melted staterespectively. A mixing tool mixes the interface material when in aplasticized state.

[0012] Preferably, the apparatus further comprises forming means forexerting force on the elements to control forming thereof.Advantageously, the temperature of the forming means is controlled toprovide heating or cooling of the elements.

[0013] In one preferred embodiment, the forming means comprises at leastone forging plate. Advantageously, the apparatus further comprisescontrol means for sensing the force exerted by the forging plate and forcontrolling feeding of the first and second elements based thereon.Preferably, the control means controls one of feed rate and travel speedto control feeding of the elements.

[0014] In another preferred embodiment, the forming means comprises aplurality of rollers. In this embodiment, the apparatus preferablyfurther comprises control means for controlling the force exerted by therollers and for controlling feeding of the first and second elementsbased thereon. Preferably, the control means controls one of feed rateand travel speed to control feeding of said elements.

[0015] Preferably, the apparatus further comprises control means forsensing energy input to the heating elements and for controlling one offeed rate or travel speed of the elements based thereon.

[0016] In a preferred implementation, the apparatus further comprises apre-weld tack welding means located upstream of the mixing tool for tackwelding the two elements together prior to mixing by the mixing tool.

[0017] Advantageously, the mixing tool is retractable so as to enablecomplete withdrawal thereof from the first and second elements.

[0018] Preferably, the apparatus further comprises containment forgingplates for containing the first and second elements during mixing by themixing tool.

[0019] According to yet another aspect of the present invention, ajoined workpiece has a first element comprising a first metal materialhaving a first plasticized temperature and a second element comprising asecond metal material having a second plasticized temperature differentfrom the first plasticized temperature. A longitudinal weld joint isformed between the first metal element and the second metal element. Theweld joint has a recrystallized fine grain matrix.

[0020] A key feature of the present invention relates to the separationof a heating process to form a plasticized or melted interface materialbetween two elements to be joined and a mixing process for mixing theinterface material together while in a plasticized state. Thisseparation enables each of the respective materials being joined to beheated and plasticized/melted independently. One advantage of thisfeature is that the invention enables dissimilar metals to be weldedtogether which previously were not able to be joined, i.e., metals suchas copper/aluminum, stainless steel/copper, and stainless steel/titaniumFor example, a copper/aluminum weld can be achieved by providingindependent temperature control as each alloy is brought up to itsrespective plasticized/melted state. A further advantage of separatingthe heating process from the weld matrix transformation process is thatcomparatively high workpiece travel rates during welding can beobtained. In this regard, the present invention is not inherentlylimited with respective to travel rates as is the case with a frictionstir welding process.

[0021] An additional important feature of the present invention is thatthe invention provides matrix transformation of the interface materialfrom dendritic to fine grained material. More specifically, the presentinvention provides for transforming grain structure from thedendritic-type weld microstructure formed as a result of the heatingprocess to a recrystallized fine, non-dendritic grain structure. As aconsequence, the resulting final grain structure is typically that of asolid state weld material and thus exhibits excellent mechanicalmaterial properties.

[0022] A further advantage of the present invention is that theinvention can provide long longitudinal welds of varying materialthickness.

[0023] Yet another advantage of the present invention is that theinvention can be used to form longitudinal welds exhibiting a solidstate weld material property without a backing anvil such as is requiredin stir welding.

[0024] Further features and advantages of the present invention will beset forth in, or apparent from, the detailed description of preferredembodiments thereof which follows.

BRIEF DESCRIPTION OF THE DRAWING

[0025]FIG. 1 is a longitudinal cross sectional view of a weldingapparatus according to a preferred embodiment of the present invention;

[0026]FIG. 2 is an end elevational view of the welding apparatus of FIG.1; and

[0027]FIG. 3 is a longitudinal cross sectional view, partially brokenaway, of a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] Referring now to FIG. 1 and FIG. 2, there is shown a weldingdevice, generally denoted 10, which is adapted to join together firstand second elements 14 and 16 of a workpiece 12. The welding device 10includes heating elements 18 and 20.

[0029] Heating element 18 provides sufficient heat to plasticize or meltthe material of element 14 and may comprise such conventional heatingdevices as lasers or plasma torches or other suitable devices known inthe art. Similarly, heating element 20 provides sufficient heat toplasticize or melt the material of element 16. As a result, together,heating elements 18 and 20 transform a portion of the solid material ofthe workpiece 12, viz., respective abutting portions of elements 14, 16to form an interface 34 (see FIG. 2) in a plasticized or melted phase,between the elements 14, 16.

[0030] The heating elements 18 and 20 can be controlled individually forproviding heating at a desired separate temperature. For example, whereelements 14 and 16 comprise dissimilar materials having differentmelting points, heating element 18, preferably provides heating at theplasticizing or melting temperature of element 14 whereas heatingelement 20 preferably provides heating at the plasticizing or meltingtemperature of the material of element 16.

[0031] The material of element 14 and element 16 may be formed of thesame or different metal material. For example, element 14 may be copperor stainless steel, and element 16 may be aluminum, copper or titanium,so that when joined together, elements 14 and 16 form a workpiece 12formed of copper/aluminum, or stainless steel/copper or stainlesssteel/titanium or another combination.

[0032] In an alternative embodiment, additional heating elements (notshown) of the type of heating elements 18, 20 may be disposed adjacentto the heating elements 18, 20 and/or below workpiece 12, to assist inplasticizing/melting the abutting portions of the elements 14, 16. Anoptional tack weld heater 22, disposed upstream, relative to the heaters18 and 20, provides sufficient heat to form an initial tack weld betweenthe elements 14, 16.

[0033] As shown in FIG. 2, a pair of clamping elements 36, 38 apply arespective force on the workpiece 12 towards each other. The forceapplied maintains elements 14 and 16 in proper alignment with eachother.

[0034] A mixing tool such as toothed grinding/extruding member 40 ispositioned in the path of the interface 34 between elements 14 and 16and rotates to mix the material of interface 34 when in a plasticizedphase. The grinding/extruding member 40 allows plasticized metal of theinterface 34 to flow and extrude through the individual teeth, which areindicated at 40 a, of member 40. The teeth 40 a are angled, thus forcingthe plasticized material to flow downwards or upwards depending upon theangle of the teeth 40 a. As a result, the grinding/extruding member 40will recrystallize the dendritic matrix structure which may have beenformed in the material of interface 34 as a result of the meltingprocess. Grinding/extruding member 40 is retractable as indicated byarrow A and can be completely withdrawn from the workpiece formed byelements 14 and 16.

[0035] A plurality of force actuators 42 are located downstream ofgrinding member 40 and used to apply a force on a pair of forging plates44, 46 which are located on opposite sides of workpiece 12. Forceactuators 42 include rollers 42 a which engage and bear against plates44, 46. In an alternative embodiment, plates 44, 46 can be eliminatedand rollers 42 a used to bear directly on the workpiece, i.e., eitherrollers, or plates, can be used separately to exert the required forceto further form the workpiece. Further, the rollers or forging platescan be heated or cooled (e.g., by water cooling) to control thetemperature of the workpiece material.

[0036] As indicated in the drawings, the heating elements describedabove and the grinding member 40 extend through forging plates 44, 46 soas to permit them to perform their respective functions. The forceexerted upon the forging plates 44, 46 by the force actuators 42 isconstant for a workpiece having constant thickness, while a variableforce is exerted on the forging plates 44, 46 by the actuators 42 toaccommodate workpieces of a tapered thickness.

[0037] A pair of motion control devices indicated separately at 48control the amount of movement of the forging plates 44, 46. The motioncontrol devices 48 each may comprise a LVDT, a laser device or othersuitable motion control device known in the art.

[0038] During the operation of the welding device 10, the workpiece 12is inserted into an entrance 50 of a housing 52 which houses the variouselements and units described above. As set forth hereinbefore, theheating elements 18, 20 provide the desired heating of elements 14 and16, to form the plasticized or melted phase interface 34.

[0039] During the heating process, undesirable dendritic-type Weldmicrostructures may be introduced into the matrix of the interfacematerial 34. Advantageously, the heating process takes place in an inertenvironment. For example, nitrogen gas can be pumped into housing 52 toprovide an inert environment, thereby reducing or eliminating theoxidation of the material of workpiece 12 during the heating process.

[0040] The workpiece 12 proceeds through the welding device 10 in adirection denoted by arrow 54. The material of interface 34 is in aplasticized or melted state while in the area generally indicated byreference numeral 56. As the workpiece 12 proceeds along direction 54,if the interface was heated to a melted state, the melted interface istransformed into a plasticized state. The transition point where themelted interface becomes plasticized is denoted by a dashed line 58. Theworkpiece 12 transitions from the melted state to the plasticized statedue to the absence of applied heat. Alternatively, if the material ofinterface 34 is merely heated to a plasticized state, the interfacematerial remains in a plasticized state as the workpiece proceeds pastdashed line 58.

[0041] The workpiece 12 then proceeds to the location thegrinding/extruding member 40. The interface 34, now in the plasticizedstate, is processed by the grinding/extruding teeth 40 a of member 40and the plasticized material of interface 34 flows and is extrudedthrough the teeth 40 a. As indicated above, this processing of theinterface material dramatically recrystallizes the grain structure, thusproducing a fine grained weld matrix when fully cooled.

[0042] The workpiece 12 next travels past the grinding/extruding member40 to a location where the hot interface material, which is still in aplasticized state, is forged under pressure by the forging plates 44,46.

[0043] In an advantageous embodiment, a controller or control system 60is employed which controls the feed rate or travel speed of theworkpiece formed by elements 14 and 16 by controlling the force exertedby either the forging plates 44, 46 and/or rollers 42 a (whether usedseparately or in combination, as indicated schematically by the dashedlines in FIG. 1). Although a separate controller 60 is shown, thecontrol system could directly control force actuators 42.

[0044] Alternatively, or in addition, a control system or controller 62is also provided which controls the material feed rate or travel speedby monitoring or sensing the energy input to the heating elements 18 and20, or as illustrated, the energy input to a further heating element 64.The overall control system could also include a sensor 66 for sensingfeed rate or travel speed and supplying a corresponding input signal tocontroller 62.

[0045] Referring to FIG. 3, in accordance with a further importantfeature of the invention, separate containment forging plates 68 and 70are provided closely adjacent to or, in one embodiment, in surroundingrelation to, the mixing tool 40 so as to contain the heated material ofelements 14, 16 during rotation of the grinding teeth 40 a of mixingtool 40.

[0046] It should be apparent to those of ordinary skill that the presentdevice and process offers important advantages over previous weldingmethods. These advantages include the ability to weld togetherdissimilar alloys which previously could not be joined due todifferences in their respective melted and plasticized phasetemperatures. Further, the separation of the plasticizing/meltingprocess and interface matrix transformation process results insignificantly enhanced welding speeds.

[0047] Although the invention has been described above in relation topreferred embodiments thereof, it will be understood by those skilled inthe art that variations and modifications can be effected in thesepreferred embodiments without departing from the scope and spirit of theinvention.

What is claimed:
 1. A welding method for joining a workpiece comprisingfirst and second elements in abutting relation along facing surfaces,said method comprising the steps of: heating the first and secondelements to plasticize or melt the elements at least at the facingsurfaces so as to form an interface therebetween of material in aplasticized or melted state; allowing the interface material, ifpreviously in a melted state, to cool from the melted state to aplasticized state; and mixing the interface material while in theplasticized state.
 2. The method of claim 1, wherein said heating stepcomprises: heating the first element to a welding temperature for thefirst element; and heating the second element to a welding temperaturefor the second element different from the welding temperature for thefirst element.
 3. The method of claim 2 wherein a first heat source isused to provide heating of the first element and a second heat source isused to provide heating of said second element.
 4. The method of claim 3wherein said first and second heat sources comprise separate lasers. 5.The method of claim 1, wherein said mixing step comprising mixing theinterface sufficiently to recrystallize dendritic matrix structuresformed in the interface material during said heating step.
 6. The methodof claim 1, further comprising the step of applying pressure along theinterface material when in the plasticized state.
 7. The method of claim1, wherein said heating step is performed in an inert environment. 8.The method of claim 1, wherein said mixing step is performed in an inertenvironment.
 9. The method of claim 1, wherein said mixing stepcomprises grinding and extruding the interface material.
 10. Anapparatus for forming a weld joint in a workpiece between first andsecond elements in abutting relation along facing surfaces, saidapparatus comprising: a heating device for plasticizing or melting thefirst and second elements at least at facing surfaces thereof so as toform an interface therebetween of interface material in a plasticized ormelted state; and a mixing tool for mixing the interface material whenin a plasticized state.
 11. The apparatus of claim 10 wherein saidheating device comprises first and second separate heat sources.
 12. Theapparatus of claim 10, wherein said mixing tool comprises a grinding andextruding device.
 13. The apparatus of claim 10, wherein said heatingdevice applies heat to the first element at a first element weldingtemperature and applies heat to the second element at a second elementwelding temperature different than the first welding temperature. 14.The apparatus of claim 13 wherein said heating device comprises a firstheat source for applying heat to said first element and a second,separate heat source for applying heat to said second element.
 15. Theapparatus of claim 14 wherein said first and second heat sources eachcomprise a laser.
 16. The apparatus of claim 10, further comprisingforging plates for applying a force to the interface material while inthe plasticized state.
 17. The apparatus of claim 10, further comprisinga housing for maintaining an inert environment for the first and secondelements when melted by said heating device.
 18. An apparatus of claim10, further comprising forming means for exerting force on said elementsto control forming thereof.
 19. An apparatus of claim 18, wherein thetemperature of said forming means is controlled to provide heating orcooling of said elements.
 20. An apparatus of claim 18, wherein saidforming means comprises at least one forging plate.
 21. An apparatus ofclaim 20, further comprising control means for sensing the force exertedby said at least one forging plate and for controlling feeding of saidfirst and second elements based thereon.
 22. An apparatus of claim 21,wherein said control means controls one of feed rate and travel speed tocontrol feeding of said elements.
 23. An apparatus of claim 18, whereinsaid forming means comprises a plurality of rollers.
 24. An apparatus ofclaim 23, further comprising control means for controlling the forceexerted by said rollers and for controlling feeding of said first andsecond elements based thereon.
 25. An apparatus of claim 24, whereinsaid control means controls one of feed rate and travel speed to controlfeeding of said elements.
 26. An apparatus of claim 10, furthercomprising control means for sensing energy input to said heating deviceand for controlling one of feed rate and travel speed of said elementsbased thereon.
 27. An apparatus of claim 10, further comprising apre-weld tack welding means located upstream of said mixing tool fortack welding the two elements together prior to mixing by said mixingtool.
 28. An apparatus of claim 10, wherein said mixing tool isretractable so as to enable complete withdrawal thereof from the firstand second elements.
 29. An apparatus of claim 10, further comprisingcontainment forging plates for containing the first and second elementsduring mixing by said mixing tool.